1. Field of the Invention
The present invention relates to an optical transmission device and an optical transmission method, and more particularly to an optical transmission device and an optical transmission method that can unequivocally establish synchronization at the receiver end in addition to preventing degradation of reception quality and enabling transmission of signals of different bit rates.
2. Description of the Related Art
In recent years, attention is being focused on optical transmission systems using optical fibers as a communication system having the potential for large volumes of data at high speed. In an optical transmission system, an optical line terminal (OLT) serving as a host station device and a plurality of optical network units (ONUs) functioning as subscriber devices are connected by optical fibers.
In the optical transmission system, here is ongoing formulation of standardization of data frame structure when data is transmitted downstream (from the OLT to the ONUs). For example, “Gigabit-capable Passive Optical Networks (G-PON)-Transmission convergence layer specification”, Recommendation G. 984.3 of International Telecommunication Union-Telecommunication (ITU-T), formulated in February, 2004, stipulates that the Physical Control Block downstream (PCBd), which is equivalent to a frame header, be followed by a data area that includes an Asynchronous Transfer Mode (ATM) cell or a G-PON Encapsulation Mode (GEM) area. Similarly, “Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications” Institute of Electrical and Electronic Engineers (IEEE) Standard 802.3 TM-2005, formulated in December, 2005, stipulates that the Multi-Point Control Protocol (PMCP) frame includes a header having a preamble and a destination address, and the data area be of variable length.
The data contained in such frames are transmitted to the ONUs corresponding to the addresses. In other words, in a frame according to Recommendation G. 984.3 of ITU-T, a frame containing data to be transmitted to a plurality of ONUs includes the addresses of the ONUs in the frame header attached to the ATM cell or the GEM area. Specifically, identifiers that point to the destination ONUs to which data is transmitted are stored as Virtual Path Identifiers (VPI) in the frame header of the ATM cell or as Port IDs in the frame headers of the GEM area. Thus, the ONUs can identify the data to be addressed to itself in accordance with the identifiers in the frame header.
As regards MPCP frame according to Standard 802.3 of IEEE, each MPCP frame contains only one ONU address, which is the destination address of the data contained in the MPCP frame. Specifically, the ONU is identified by a Logical Link Identifier (LLID) stored in the preamble of the frame header. Then, the ONU determines the MPCP frame having the destination address of the data applicable to itself and receives the MPCP frame.
However, the bit rate of the frames has not been taken into consideration while formulating these standardizations. In other words, the standardizations have been made on the assumption that data is transmitted and received by all the ONUs at the same bit rate. Therefore, problems will be encountered if there are signals of diverse bit rates to be transmitted from one OLT to a plurality of ONUs.
As a solution to the problem, a technology is disclosed in Japanese Patent Application Laid-open No. 2000-188593 whereby the OLT stores in the frame header of each frame speed data and phase data concerning each sub-frame contained in the frame, and each ONU receives the sub-frames having the speed data and phase data applicable to itself. Thus, even if there are signals with different bit rates meant for a plurality of ONUs, each ONU receives the correct signal meant for it.
As described above, it is assumed that normally transmission of signals from the OLT to a plurality of ONUs takes place at the same bit rate. However, if the bit rates of the signals meant for some of the ONU addresses are increased, malsynchronization results in the concerned ONUs. Specifically, each ONU extracts a clock component from the received signal and performs a Clock Data Recovery (CDR) process to establish synchronization. If the bit rate of the received signal is stepped up from the bit rate at which the ONU is capable of properly receiving the signal, the ONU will not be able establish a normal synchronization and retrieve the identifier included in the header. Not being able to retrieve the identifier results in the ONU not being able to receive the data meant for it.
Further, the signal with a bit rate other than that can be received by the ONU is perceived as noise. Therefore, if positions of such signals are not identified accurately, there could be degradation in the reception quality.
Further, in the technology disclosed in Japanese Patent Application Laid-open No. 2000-188593, whereby the ONUs can be identified by the speed data and phase data, every ONU is required to have a different bit rate in order to be clearly identified in accordance with the speed data and phase data. In other words, all the ONUs connected to the OLT in such an optical transmission system will be required to have different bit rates, resulting in a tradeoff in flexibility in system designing.